Loudspeaker



Feb. 26, 1963 M. w. WIDENER LOUDSPEAKER 2 Sheets-Shem 1 Filed March 1,1961 MAME/CE W Mom/2 III I l3 INVENTOR.

BY /6Mm/i% ATTOF/i/E) Feb. 26, 1963 w. WIDENER 3,079,471

LOUDSPEAKER Filed March .1. 1961 2 Sheets-Sheet 2 III-16,4:

Maw/c5 kill 1405mm IN V EN TOR.

,dMPL/F/EES y W United States Patent Ofilice Fatenteti Fold. 26, 19%3 onof rest, No. sales T his invention relates to loudspeakers, andparticularly to loudspeakers having diaphragms movable byelectromagnetic means engaging the peripheries thereof.

in the noted art it has been recognized that 2. diaphragm moved by meansengaging the periphery thereof can give improved performance if a verylarge part of the diaphragm is unyieldingly rigid so as to displace theair more uniformly to create the sound waves desired. The larger is therigid area of the diaphragm, the greater is the volume of air displacedon each vibrational stroke, and the greater is the average amplitude ofdisplacement. Also, each such diaphragm is limited in range by an upperresonant frequency at which the rigidity of the element is overcome andat which it begins to undulate or flex uncontrollably so as to produceundesirable distortions in the sound produced. Thus, the more rigid thediaphragm, the wider is the frequency range across which the speaker canbe used without resonance. In effect, it is desired to n'rake thediaphragm more like the solid cad of a piston and less like the flexiblehead of a drum.

Previously, attempts to design a speaker embodying such a diaphragm havebeen limited by the circumstance that high audible frequencies require alow-mass diaphragm because very quick alternating resonse and reversalof movement of the diaphragm is needed to produce these frequencies.Assuming a given material of high strength and low unit mass (e.g.,aluminum), it is nevertheless true that the lower the total mass, thethinner th diaphragm must be, and the less rigid.

Until now, no speaker is known that can reproduce the higher audiblefrequencies without being limited in range on the lower side atapproximately 3000-4000 cycles per second.

Accordingly, it is an object of this invention to provide a loudspeakerhaving a non-resonant frequency range encompassing substantially all ofthe desirable audible fret encies above approximately 500 cycles persecond.

lit is another object of the invention to provide a loudspeaker having aresponse flatness range of novel extent and of improved constancy.

it is still another object of the invention to provide a loud peaker ofimproved performance and capable of being manufactured in simple andinexpensive form for economical sale and use.

A loudspeaker in accordance with the present invention satisfies theforego and other purposes by making use of a diaphragm in the shape ofan oblate dome having increased resistance to distortion under forcesacting in an axial direction of the dome, and maximum rigidity inproportion to mass; and this diaphragm is acoustically coupled to asound box the cavity of which is tuned to a lower resonant frequencythan the free resonant frequency of the mass of the moving parts and thesuspension thereof.

Other objects and advanta es will be explained in the followingspecification, considered together with the accompanying drawing, inwhich:

FlGURE l is a cross-sectioned perspective elevation view of aloudspeaker constructed in accordance with the invention;

FlGURE 2 is a fragmentary broken-away plan view, to an enlarsed scale,taken substantially on the plane of line 2-42 of FEGURE l;

FIGURE 3 is an enlarged View of that portion of FIGURE 1 enclosed withinthe arcuate line 3.--3 thereof.

FEGURE 4 is a schematic view illustrating the shape of a domeconstructed in accordance with the invention.

Referring now to FZGURE 1 there is shown a lower housing ll. defining acylindrical cavity 12 and having a bottom wall 13 reinforced by flangesE4, the wall 13 being provided with a small vent opening 16. in theupper portion of the housing 11 there is seated and firmly affixed upperhousing 17 provided with an inwardly directed flange 18; at the lowerend thereof, the flange defining a central opening 19. On the flange isafiixed a toroidal permanent magnet 21 formed, for example, of Alnica,and having a frusto-conical outer surface 22. The housing 17 is part ofone of the pole pieces of the magnet 21 and is preferably formed of softiron. The central opening 23 of the magnet is in registry with theopening 19 of flange l8. Surmounting the magnet 21 is a soft iron polepiece 24, having a toroidal shape and defining an opening 26 registeringwith the opening 2% of the magnet. On top of the housing 17 and in thesame plane with pole piece 24 is affixed a toroidal cover plate 127,which is also part of the pole piece represented by the housing 317 andis also made of soft iron. The plate 27 defines a central opening 28, sothat the cover plate circuniferentially encompassing the pole piece 24is spaced therefrom to define the opening 28 as a space between thecover plate and pole piece. A ring 29 of electrically insulatingmaterial is afiixed to the upper portion of the cover plate 27 to form aportion of the upper wall of the acoustical cavity through whichelectrical leads are to be passed, as described below. A diaphragmmounting ring Edi is aiiixed to the upper portion of the ring 29 andmounts a resilient collar 33;, which has a wavy cross-sectional shape topermit motion. The collar 3% in turn serves as a mounting for a thinaluminum diaphragm 3.2 in the shape of an oblate dome oriented with theconcave side inward and communicating with the central openings of thepole pieces 2 and magnet 21. Also afiixed to the collar 31 is adownwardly extending cylinder 33 formed, for example, of stiff paper onwhich is mounted an electrical coil The cylinder 33 is extended todispose the coil 34 (best seen in FEGURE 3) within the space 28 etweenthe cover plate 27 and the pole piece 24. A plug 36 made of acousticaldamping material such as phenolic bonded fiberglass is mounted in thecentral portion of the magnet opening 23 so as to define within theinterior of the apparatus the lower cavity 12 and an upper cavity 37. Adamping element 3%, made of solid non-magnetic material, is centrallymounted as by means of a spider on the pole piece and proiectingupwardly in the upper cavity to a position in which the upper surface ofthe element is spaced closely beneath the peak portion of the domeshaped diaphragm 512, the surface ll being formed to substantially thesame conformation as the confronting peak portion of the diaphragm. Apair of electrical leads 4-2 and 43 are arranged projecting through theinsulating ring 29 to be coupled with the coil dd, as hereinafterdescribed.

it is here noted that the cavities l2 and 3'7 are completely sealed toform what would be, in the absence of the vent opening 16, an air-tightchamber. Under some conditions of use or during shipping as by aircraft,there would be danger of the louds eaker blowing up. The opening to isprovided to circumvent such happenings.

The structure described above is further illustrated in PlGURE 2,together with a protective cover 4-3 of rigid, acoustically transparentnet material in the shape of a dome and aiiixcd to the outer portion ofring 30 to cover the diaphragm 32.

Referring now to FIGURE 3, there is shown the electrical lead 42extending through the insulating ring 29 and electrically coupled to aflexible strip Sl of electrical conducting material having undulations,one end of which is extended to pass through a hole 52 in the papercylinder 33 and is electrically coupled to an extending end 53 of theconductor forming the coil 34.

Referring now to FIGURE 4, one method for establishing thecross-sectional shape of the diaphragm 32 is illustrated. The shape isthat of an exponential curve, and particularly a symmetrical curve orhaving an axis 62 of symmetry. To construct one-half of the curve, acircular are 63 is struck from the axis 62 and from a center 64 on theaxis 62. The arc so has a predetermined angle define-d by a radius 66,the angle in this example being five degrees. From a second center 67,lying on the radius on and spaced from center 64 for a distance equal tone-tenth the length of radius 66, a second are 68 is laid oil from arc63. The arc as is also of five degrees and is defined by an outer radius69. A third are 71 also of five degrees and of radius 72 equal to 6.9 ofradius 69 is then laid off. The process is continued until the tangent'74 to the curve 61 at the outermost portion thereof lies at an angle ofapproximately sixty degrees to a line 76 that is perpendicular to theaxis as; that is to say until the tangent '74.- lies at an angle or"approximately thirty degrees to the axis 63. It will be seen that eachpair of adjacent arcs have a common tangent at the juncture of the arcs,and form a smooth curve.

In this figure, the are 81 of a circle centered on axis 2 and passingthrough the peak point 82 and the outermost point 83 of the curve oil,is shown for comparison. It will be noted that the curve 61 has a muchsharper degree of curvature (i.e., shorter radius of curvature) in thevicinity of point 83 than has the are ill; that is, the tangent 74 ofthe curve 61 at point 83 is substantially closer to being parallel withthe axis 62 the would be the tangent to the are 31 at point 33.

In operation, an electrical signal representing the wave fluctuations ofsound is supplied to the electrical leads 4?; and 43, and the varyingcurrent passing through the coil 3 causes the coil together with thecylinder 33 and dome 32 to be rapidly displaced upward and downward withespect to the magnet 21 and pole pieces 24. The wavy shape of theconductor 51 and of the collar 31 permits substantial extension of theseelements in an axial direction of the apparatus so as to impose theleast possible resistance to the displacement required at the greateramplitudes, and so as to prevent fatigue and wearing out of the twoparts. The damping element 38 helps to reduce the amplitude of vibrationof the diaphragm ll at the highest frequencies of vibration so as tocontrol the quality of sound emitted. In part, the element 38 effectsdamping as by serving as an anvil against which air displaced by thediaphragm 32 in downward movement is compressed, the space between theelement 38 and the diaphragm being relatively restricted so as to hinderthe escape of toward the periphery of element This eil'ect is alsooperative during upward movement of the diaphragm 32, because the spaceavailable for radial inward rush. of air is also relatively restricted.The presence of the plug as a dividing portion between the upper and thelo: er acoustical cavities also helps to restrict the displacement ofair betw en the cavities at the lower frequencies of vibration and so toimprove the acoustical quality of the reproduced sound.

The cross-sectional shape of the diaphragm has an important effect inincreasing the stiffness rigidity of the dome so that equivalentrigidity can be obtained with less mass of material, and converselysothat with a given mass of material a greater rigidity can be obtained,and the range of frequencies that can be reproduced is extended at theupper limit.

it will be understood that the condition of least rigidity for adiaphragm is that in which the diaphragm is hat; and that the conditionof greatest rigidity would be that in which the diaphragm most closelyapproached being in the shape of a cylindrical wall, but a cylindricalwall would have no cross-sectional surface area suitable for displacingair in an axial direction. The provision of a spherical dome, as knownin the art, is a partly successful compromise, but the oblate shape ofthe dome used in the present invention more closely approaches an idealform. As illustrated in FIGURE 4, the oblate shape of the present domeprovides a resolution of forces more nearly approaching parallelism withthe axis of the diaphragm at the peripheral portions thereof, where thegreatest strain is focused when the diaphragm is suddenly moved as byforces applied to its periphery. The mass of the dome, and the inertiaof this mass, is more readily moved without distortion of the dome whenthe dome has a more sharply acute curvature with respect to the axis atthe point where the moving forces are applied. On the other hand, at theportions of the dome near the axis where the curvature is flatter thanit would be in a spherical section, the mass remaining radially inwardof any selected circular section of the dome is comparatively quitesmall and the eilcct of the increased flatness is not deleterious. Inshort, the dome of the invention has a smaller radius of curvature inthe portions thereof that lie at a greater radius from the axis andwhere greater rigidity 'is needed to withstand the inertia forces of thecentral or axial portion of the dome, and has greater flatness near theaxial portion where the strength does not have to be so great to producerigidity.

The range of non-resonant response of the diaphragm having beenbroadened by the construction above described, the loudspeaker isadditionally tuned as by caretul dimensioning and proportioning of thecavities 12 and 37 so that the low r resonant frequency of the coupledcavities is lower than the frequency of tree resonance of the diaphragmand moving parts attached thereto, so that the speaker as a whole islimited in its response range only by the lower resonant frequency ofthe cavities and by the upper resonant frequency of the diaphragm.

In a preferred example of the device, constructed and proportionedsubstantially as illustrated in the drawing, a response flatness rangeof i1 decibel has been achieved between the frequencies of 500 cyclesper second and 15,009 cycles per second.

if desired, as shown in FIGURE 5, and as an alternative to the use ofdamping element 38 for damping the resonance of the speaker, the leadmay be coupled to a clamping circuit 4d, comprising an inductor 35, acapacitor 4e and a resistor 47 connected in parallel, the circuit 3being in series between the coil and a constant-voltage amplifier (notshown).

Thus, there has been described in the foregoing a wide frequency rangeloudspeaker in which an acoustical chamber is coupled to a diaphragm inthe shape of an oblate dome oriented with the concave side facing thechamber, so as to provide a very wide range of nonresonant frequencyresponse.

What is claimed is:

1. A wide frequency range loudspeaker comprising: a housing defining anacoustical chamber closed at one end; a diaphragm mounted closing theother end of sai chamber and arranged for axial movement relative tosaid housing to cause air pressure variations within and without saidchamber, said diaphragm having a flexible eripheral portion secured tosaid housing and a rigid central portion in the shape of an oblate domeoriented with the concave side facing said chamber; and electromagneticIncans engaging the periphery of said dome portion for eilectiru axialmotion of said dome portion in response to variations of a controlsignal so as to pro duce corresponding sound waves.

2. A loudspeaker as defined in claim 1, wherein the cross-sectionalshape of said dome is defined by an exponential curve.

3. A Wide frequency range loudspeaker comprising: a housing defining anacoustical chamber closed at one end; a diaphragm mounted closing theother end of said cavity and arranged for axial movement relative tosaid housing to cause air pressure variations Within and without saidchamber, said diaphragm having a flexible peripheral portion secured tosaid housing and a rigid central portion in the shape of an oblate domeoriented with the concave side facing said chamber, said dome having across-sectional shape characterized by a symmetrical curve having anaxis of symmetry, each half of said curve having a plurality of adjacentarcuate portions extending from said axis outwardly, each pair ofadjacent portions having a common tangent at the juncture thereof, theradius of each portion being a predetermined tunction of the radius ofthe adjacent axially inward portion, the angle of each portion being apredetermined function of the angle of the adjacent axially inwardportion, and the tangent to the outermost part of re outermost portionbeing at an inclination of approximately thirty degrees to said axis ofsymmetry; and electromagnetic means engaging the periphery of said domeportion for eitecting axial motion of said dome portion in response tovariations of a control signal so as to produce corresponding soundwaves.

4. A wide frequency range ioudpeaker comprising: a housing defining anacoustical chamber closed at one end; a diaphragm mounted closing theother end of said cavity and arranged for axial movement relative tosaid housing to cause air pressure variations within and without saidchamber, said diaphragm having a rigid central portion in the shape ofan oblate dome oriented with the concave side facing said chamber, and aflexible peripheral portion of substantial radial width constituting acantilever mounting for said dome, whereby said dome is axially movablefor maximum excursion with minimum unit distortion of said peripheralportion; and electromagnetic means engaging the periphery of said domeportion for effecting axial motion of said dome portion in response tovariations of a control signal so as to produce corresponding soundwaves.

5. A wide frequency range loudspeaker com-prising: a housing defining anacoustical chamber closed at one end; a diaphragm mounted closing theother end of said cavity and arranged for axial movement relative tosaid housing to cause air pressure variations within and Without saidchamber, said diaphragm having a flexible peripheral portion secured tosaid housing and a rigid cen tral portion in the shape of an oblate domeoriented with the concave side facing said chamber; electromagneticmeans engaging the periphery of said dome portion for effecting axialmotion of said dome portion in response to variations of a controlsignal so as to produce corresponding sound waves; and said chamberbeing tuned to a resonant frequency lower than the free resonantfrequency of said dome together with the moving parts attached thereto.

6. A wide freouency range loudspeaker comprising: a housing defining anacoustical chamber closed at one end; a diaphragm mounted closing theother end of said cavity and arranged for axial movement relative tosaid housing to cause air pressure variations within and without saidchamber, said diaphragm having a flexible peripheral portion secured tosaid housing and a rigid central portion in the shape of an oblate domeoriented with the concave side facing said chamber; said chamber beingtuned to a resonant frequency lower than that of said dome;electromagnetic means engaging the periphery of said dome portion foreffecting axial motion of said dome portion in response to variations ofa control signal so as to produce corresponding sound waves; and aquantity of acoustical damping material disposed in said chamberintermediate the ends thereof.

7. A Wide frequency range loudspeaker comprising: a housing defining anacoustical chamber closed at one one; a diaphragm mounted closing theother end of said cavity and arranged for axial movement relative tosaid housing to cause air pressure variations within and without saidchamber, said diaphragm having a flexible peripheral portion secured tosaid housing and a rigid central portion in the shape of an oblate domeoriented with the concave side facing said chamber; electromagneticmeans engaging the periphery of said dome portion for effecting axialmotion of said dome portion in response to variations of a controlsignal so as to produce corresp acting sound waves; and a dampingelement mounted Within said chamber, said element having a portionfacing said dome and spaced therefrom so as to restrict the air fiowimmediately behind said dome when said dome is axially moved.

8. A wide frequency range loudspeaker comprising: a housing defining achamber closed at one end; a toroidal magnet mounted co-axially withinsaid housing and spaced from said closed end so as to define a first endcompartment of said chamber communicating with the central opening ofsaid toroidal magnet; a diaphragm covering the other end of said housingand spaced from said magnet so as to define a second end compartment ofsaid chamber communicating with said central opening of said toroidalmagnet, said diaphragm having a flexible peripheral portion secured tosaid housing and a rigid central portion in the shape of an oblate domeoriented with the concave side facing said chamber; an electrical coilco-axially atiixed to the peripheral portion of said dome and extendingtoward said magnet; and means for supplying electrical current to saidcoil, whereby said coil is attracted to and repulsed from said magnet inresponse liO variations in the strength and direction of said current,and whereby said dome is conjointly displaced to generate sound waves inthe air within and without said chamber.

9. A wide frequency range loudspeaker comprising: a housing defining achamber closed at one end; a toroidal magnet mounted co-axially withinsaid housing and spaced from said closed end so as to define a first endcompartment of said chamber communicating with the central opening ofsaid toroidal magnet; a diaphragm. covering the other end of saidhousing and spaced from said magnet so as to define a second endcompartment of said chamber communicating with said central opening ofsaid toroidal magnet, said diaphragm having a flexible peripheralportion secured to said housing, said peripheral portion being ofsubstantial radial Width and having a plurality of concentric folds forexpansion of said peripheral portion in either axial direction, saiddiaphragm also having a rigid central portion in the shape of an oblatedome oriented with the concave side facing said chamber; an electricalcoil co-axiaiiy afiixed to the periphery of said dome and extendingtoward said magnet; and electrical leads for supplying electricalcurrent to said coil, said leads being mounted to extend radially inwardthrough said housing and being affixed at the ends thereof to the endsof said coil so as to form an electrical connection therewith, theportions of said leads tending between said housing and said coil beingrovided with a plurality of transverse folds for resilient extension ofsaid leads upon axial movement of said coil; whereby said coil isattracted to and repulsed from said magnet in response to variations inthe strength and direction of said current, and whereby said dome isconjointly displaced to generate sound waves in the air within andWithout said chamber.

10. A wide frequency range loudspeaker comprising: a housing defining achambe closed at one end; a toroidal magnet mounted co-axially withinsaid housing and spaced from said closed end so as to define a first endcompartm at of said chamber communicating with the central opening ofsaid toroidal magnet; a diaphragm covering sc ee the other 3rd of saidhousing and spaced from said net so as to define a second endcompartment of said chamber communicating with said central opening ofsaid toroidal magnet, said diaphragm having a flexible peripheraiportion secured to said housing, said peripheral portion being ofsubstantial radial Width and having a plurality of concentric folds forexpansion of said peripheral portion in either axial direction, saiddiaphragm also having a rigid central portion in the shape of an oblatedome oriented with the concave side facing said chamber; an electricalcoil co-axially afiixed to the peripheral portion of said dome andextending toward said magnet; a pair of electrical leads for supplyingelectrical current to said coil, said leads being mounted to extendradially inward through said housing and being atlixed at the ends tothe ends or" said coil so as to form an electrical connection therewith,the portions of said leads extending between said housing and said coilbeing provided with a plurality of transverse folds for resilientextension of said leads upon axial movement of said coil; a quantity ofacoustical damping material mounted in the central opening of saidtoroidal magnet; and a damping element mounted within said second endcompartment, said element being shaped to conform to the concave side ofsaid dome and being spaced therefrom so as to restrict the flow of airimmediately behind said dome when said dome is axially moved, saidelement being mounted by means of a spider attached thereto and to theadjacent portions of said toroidal mag net, whereby said coil isattracted to and repulsed from said magnet in resnonse to variations inthe strength and direction of said current, and whereby said dome isconjointly displaced to generate sound Waves in the air within andwithout said chamber.

References tjited in the file of this patent UNlTED STATES PATENTS1,812,389 Wcnte June 30, 1931 1,926,187 Young Sept. 12, 1933 2,047,777Hartmann July 14, 1936 2,141,595 CornWell Dec. 27, 1938 2,238,741Laufier Apr. 15, 1941 2,549,963 De Boer et a1. Apr. 24, 1951 2,848,561Gorilte Aug. 19, 1958 2,971,597 Gorike Feb. 14, 1961 FOREIGN PATENTS NR.190,987 Austria July 25, 1957 reat Britain Dec. 19, 1956

1. A WIDE FREQUENCY RANGE LOUDSPEAKER COMPRISING: A HOUSING DEFINING ANACOUSTICAL CHAMBER CLOSED AT ONE END; A DIAPHRAGM MOUNTED CLOSING THEOTHER END OF SAID CHAMBER AND ARRANGED FOR AXIAL MOVEMENT RELATIVE TOSAID HOUSING TO CAUSE AIR PRESSURE VARIATIONS WITHIN AND WITHOUT SAIDCHAMBER, SAID DIAPHRAGM HAVING A FLEXIBLE PERIPHERAL PORTION SECURED TOSAID HOUSING AND A RIGID CENTRAL PORTION IN THE SHAPE OF AN OBLATE DOMEORIENTED WITH THE CONCAVE SIDE FACING SAID CHAMBER; AND ELECTROMAGNETICMEANS ENGAGING THE PERIPHERY OF SAID DOME PORTION FOR EFFECTING AXIALMOTION OF SAID DOME PORTION IN RESPONSE TO VARIATIONS OF A CONTROLSIGNAL SO AS TO PRODUCE CORRESPONDING SOUND WAVES.